1. Field of the Invention
The present invention relates to an automatic gain control circuit. More particularly, the present invention relates to an automatic gain control circuit for controlling an amplitude level of a reproduced signal obtained from a recording medium such as an optical disk or a magnetic disk to be at a constant level.
2. Description of the Background Art
In a reproduced signal processing circuit of an optical disk apparatus, laser light is illuminated onto pits on a disk, and the reflected light is picked up by an optical pickup and is converted into an electric signal, thereby obtaining a reproduced signal. The reproduced signal thus obtained is amplified by a preamplifier, and then the amplitude level thereof is adjusted by a variable gain amplifier. In the variable gain amplifier, the amplitude level of the reproduced signal is adjusted so as to be matched with the dynamic range of an A/D converter in a subsequent stage. The reproduced signal, whose amplitude level has been adjusted by the variable gain amplifier, is subjected to a waveform equalization process, and then quantized by the A/D converter. The reproduced signal, which has been quantized, is decoded by a digital processing section into binary reproduced data.
In the reproduced signal processing circuit, the gain of the variable gain amplifier is controlled as follows in order to match the amplitude level of the reproduced signal with the dynamic range (input voltage range) of the A/D converter. First, the reproduced signal, which has been subjected to a waveform equalization process, is subjected to a full wave rectification process, and then passed through a low-pass filter so as to smooth the waveform peaks. Thus, the amplitude level of the reproduced signal is derived. Then, the derived amplitude level is compared with a reference level that corresponds to the dynamic range of the A/D converter so as to calculate the error therebetween. Then, a gain control signal having a voltage level according to the calculated error is generated. The variable gain amplifier amplifies the reproduced signal with a gain according to the voltage level of the gain control signal. Thus, the gain of the variable gain amplifier is controlled according to the error between the amplitude level of the reproduced signal and the reference level, whereby the reproduced signal, which has been amplified by the variable gain amplifier, has an amplitude level that is matched with the dynamic range of the A/D converter in a subsequent state.
In the automatic gain control circuit, it is required to adjust the amplitude level of the reproduced signal so that the quantization process can be performed efficiently in the A/D converter in a subsequent stage, and to adjust, in real time, the amplitude level of the input reproduced signal. However, it is expected that the disk read speed will be increased and the wavelength of the light source will be shortened in the future, whereby the SN ratio of the reproduced signal may deteriorate and the high frequency component thereof may be weakened. The deterioration in the SN ratio of the reproduced signal and the weakening of the high frequency component thereof will make the output of the variable gain amplifier unstable.
An object of the present invention is to provide an automatic gain control circuit in which a stable output can be obtained from a variable gain amplifier.
According to the present invention, an automatic gain control circuit includes a variable gain amplifier, an A/D converter, a clock generation circuit, an error calculation section and a control signal generation circuit. The variable gain amplifier amplifies a reproduced signal from a recording medium. The A/D converter quantizes the reproduced signal, which has been amplified by the variable gain amplifier. The clock generation circuit generates a clock that is synchronized with the reproduced signal, which has been quantized by the A/D converter. The error calculation section generates an error signal according to a difference between the reproduced signal, which has been quantized by the A/D converter, and one of n values (n is a positive integer) that corresponds to the reproduced signal, which has been quantized by the A/D converter, the n values being determined by a recording constraint and a recording format for the recording medium and a method for weighting the reproduced signal from the recording medium. The control signal generation circuit generates a gain control signal based on the error signal generated by the error calculation section. The A/D converter quantizes the reproduced signal, which has been amplified by the variable gain amplifier, in synchronism with the clock from the clock generation circuit. The variable gain amplifier amplifies the reproduced signal from the recording medium with a gain according to a level of the gain control signal from the control signal generation circuit.
In the automatic gain control circuit described above, not only the peak values of the reproduced signal, which has been quantized by the A/D converter, but also the intermediate values thereof, are used in the generation of the error signal. Therefore, as compared to a case where only the peak values are used, the output of the variable gain amplifier is more stable.
It is preferred that the error calculation section generates the error signal when one of the n values that corresponds to the reproduced signal, which has been quantized by the A/D converter, is a desired value.
With the automatic gain control circuit described above, the error signal can be generated while selectively using only one or more of the n values whose difference from the reproduced signal has a small variation.
It is preferred that when the clock from the clock generation circuit is not synchronized with the reproduced signal, which has been quantized by the A/D converter, the error calculation section generates the error signal according to a difference between a peak of the reproduced signal, which has been quantized by the A/D converter, and a predetermined reference value, whereas when the clock from the clock generation circuit is synchronized with the reproduced signal, which has been quantized by the A/D converter, the error calculation section generates the error signal according to a difference between the reproduced signal, which has been quantized by the A/D converter, and one of n values (n is a positive integer) that corresponds to the reproduced signal, which has been quantized by the A/D converter, the n values being determined by a recording constraint and a recording format for the recording medium and a method for weighting the reproduced signal from the recording medium.
It is preferred that the predetermined reference value is an upper or lower limit value of a dynamic range of the A/D converter.
In the automatic gain control circuit described above, when the clock from the clock generation circuit is not synchronized with the reproduced signal, which has been quantized by the A/D converter, the level of the reproduced signal, which has been quantized by the A/D converter, changes irregularly. Nevertheless, the peak of the reproduced signal can be detected. Therefore, the error calculation section generates the error signal according to the difference between a peak value of the reproduced signal, which has been quantized by the A/D converter, and the upper or lower limit value of the dynamic range of the A/D converter. Then, the control signal generation circuit generates the gain control signal based on the error signal. The variable gain amplifier amplifies the reproduced signal from the recording medium with a gain according to the gain control signal. In this way, the difference between the peak value of the reproduced signal, which has been quantized by the A/D converter, and the upper or lower limit value of the dynamic range of the A/D converter is reduced. Thus, the amplitude level of the reproduced signal amplified by the variable gain amplifier is brought closer to the dynamic range of the A/D converter.
When the clock from the clock generation circuit is synchronized with the reproduced signal, which has been quantized by the A/D converter, the reproduced signal, which has been quantized by the A/D converter, takes a level that is substantially equal to one of n values. The error calculation section generates the error signal according to the difference between the reproduced signal, which has been quantized by the A/D converter, and one of the n values that corresponds to the reproduced signal. Then, the control signal generation circuit generates the gain control signal based on the error signal. The variable gain amplifier amplifies the reproduced signal from the recording medium with a gain according to the gain control signal. In this way, the amplitude level of the reproduced signal, which has been quantized by the A/D converter, is adjusted to a level that is matched with the dynamic range of the A/D converter. Thus, not only the peak values of the reproduced signal, which has been quantized by the A/D converter, but also the intermediate values thereof, are used in the generation of the error signal. Therefore, as compared to a case where only the peak values are used, the output of the variable gain amplifier is better matched with the dynamic range of the A/D converter and is more stable.
As described above, in the automatic gain control circuit, when the clock from the clock generation circuit is not synchronized with the reproduced signal, which has been quantized by the A/D converter, an error signal according to the difference between the peak value of the reproduced signal, which has been quantized by the A/D converter, and the upper or lower limit value of the dynamic range of the A/D converter is generated. In this way, until the clock from the clock generation circuit is synchronized with the reproduced signal, which has been quantized by the A/D converter, the amplitude level of the output of the variable gain amplifier can be brought somewhat closer to the level that is matched with the dynamic range of the A/D converter. As a result, it is possible to reduce the amount of time required for bringing the amplitude level of the output of the variable gain amplifier to a level that is matched with the dynamic range of the A/D converter and is stable.
It is preferred that the predetermined reference value is a value that is smaller than an upper limit value of a dynamic range of the A/D converter and greater than a median value thereof, or a value that is greater than a lower limit value of the dynamic range of the A/D converter and smaller than the median value thereof.
With the automatic gain control circuit described above, it is possible to prevent the amplitude level of the output of the variable gain amplifier from exceeding the dynamic range of the A/D converter.
It is preferred that when the clock from the clock generation circuit is not synchronized with the reproduced signal, which has been quantized by the A/D converter, the error calculation section generates the error signal according to a difference between a peak of the reproduced signal from the variable gain amplifier and a predetermined reference value, whereas when the clock from the clock generation circuit is synchronized with the reproduced signal, which has been quantized by the A/D converter, the error calculation section generates the error signal according to a difference between the reproduced signal, which has been quantized by the A/D converter, and one of n values (n is a positive integer) that corresponds to the reproduced signal, which has been quantized by the A/D converter, the n values being determined by a recording constraint and a recording format for the recording medium and a method for weighting the reproduced signal from the recording medium.
With the automatic gain control circuit described above, until the clock from the clock generation circuit is synchronized with the reproduced signal, which has been quantized by the A/D converter, the amplitude level of the output of the variable gain amplifier can be brought somewhat closer to the level that is matched with the dynamic range of the A/D converter. As a result, it is possible to reduce the amount of time required for bringing the amplitude level of the output of the variable gain amplifier to a level that is matched with the dynamic range of the A/D converter and is stable.
It is preferred that when the clock from the clock generation circuit is not synchronized with the reproduced signal, which has been quantized by the A/D converter, the control signal generation circuit generates the gain control signal based on a product between the error signal generated by the error calculation section and a first multiplier, whereas when the clock from the clock generation circuit is synchronized with the reproduced signal, which has been quantized by the A/D converter, the control signal generation circuit generates the gain control signal based on a product between the error signal generated by the error calculation section and a second multiplier that is smaller than the first multiplier.
With the automatic gain control circuit described above, when the clock from the clock generation circuit is not synchronized with the reproduced signal, which has been quantized by the A/D converter, the gain control signal is generated based on the product between the first, greater, multiplier and the error signal. Therefore, the variable gain amplifier amplifies the reproduced signal from the recording medium with a gain that is greater than that when the gain control signal is generated based on the second multiplier. In this way, the amplitude level of the output of the variable gain amplifier can be brought roughly to the dynamic range of the A/D converter. On the other hand, when the clock from the clock generation circuit is synchronized with the reproduced signal, which has been quantized by the A/D converter, the gain control signal is generated based on the product between the second, smaller, multiplier and the error signal. Therefore, the variable gain amplifier amplifies the reproduced signal from the recording medium with a gain that is smaller than when the gain control signal is generated based on the first multiplier. In this way, the amplitude level of the output of the variable gain amplifier is finely adjusted to a level that is matched with the dynamic range of the A/D converter and is stable.
As described above, in the automatic gain control circuit, when the clock from the clock generation circuit is not synchronized with the reproduced signal, which has been quantized by the A/D converter, the gain control signal is generated based on the product between the first multiplier and the error signal, whereas when the clock from the clock generation circuit is synchronized with the reproduced signal, which has been quantized by the A/D converter, the gain control signal is generated based on the product between the second multiplier and the error signal. Thus, the amplitude level of the output of the variable gain amplifier can be adjusted effectively.
It is preferred that the control signal generation circuit changes the second multiplier depending on a format of data recorded on the recording medium.
With the automatic gain control circuit described above, the amplitude level of the output of the variable gain amplifier can be adjusted effectively according to the format of the data recorded on the recording medium.